The present invention relates to a porous film, a process for producing the same and absorbent sanitary articles. Particularly, the present invention relates to a porous film composed of polyolefin resin, a filler and an ester of dipentaerythritol having 0 to 70% residual OH groups, which has a good appearance and feeling, excellent strength and proper moisture permeability, a process for producing the porous film with a high efficiency, and absorbent sanitary articles such as a disposable diaper or a sanitary absorbent (napkin) using the porous film. Further, the porous film can be applied to various uses such as clothing, packaging media, battery separators, filtering media and sanitary and medical materials.
Many attempts to uniaxially or biaxially stretch the film obtained by mixing polyolefin resin with a filler and then melt-molding the obtained mixture have been previously made as means for producing porous films. However, in these porous films, the uniaxially stretched film still has problems with respect to the anisotropy in the physical properties thereof, particularly the tear strength in the longitudinal direction (stretching direction), and the surface strength. The biaxially stretched film has no problem with respect to the anisotropy in the physical properties thereof, but does have problems with respect to the stretching property and the surface strength. In addition, both films generally show a high stiffness which is a disadvantage according their uses.
A method of improving the anisotropy in the physical properties of a film and the surface strength thereof is stretching with as low a ratio as possible to form a porous film. A method in which a low-melting point polymer, a rubber substance, a plasticizer or a surfactant is added can be considered as a method of giving flexibility to the film. However, no porous film which satisfies the balance of physical properties of molding stability, porosity, stretching property, mechanical properties such as tear strength and tensile strength, and surface strength has been found.
As a method for improving the aforementioned disadvantages of conventional porous films, a composition obtained by mixing polyolefin resin, a filler and liquid polybutadiene or liquid polybutene (Japanese Patent Application Laid-Open (KOKAI) No. 57-47334) and a composition obtained by mixing linear low-density polyethylene, a filler and a polyhydroxy saturated hydrocarbon (Japanese Patent Application Laid-Open (KOKAI) No. 58-15538) have been proposed.
In addition, a method of using a polyester plasticizer such as an adipate or an epoxy plasticizer such as epoxidized soybean oil has also been proposed in Japanese Patent Application Laid-Open (KOKAI) No. 61-144331(1986), methods of using triglyceride have been proposed in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-10141(1987) and 62-18435(1987), and a method of using silicone oil has been proposed in Japanese Patent Application Laid-Open (KOKAI) No. 62-129321(1987).
Methods of using ester compounds have also been proposed in Japanese Patent Application Laid-Open (KOKAI) Nos. 61-235439(1986), 62-18435(1987), 62-27438(1987), 62-280234(1987), 62-280235(1987), 62-288640(1987) and 63-22844(1988).
However, none of these methods has achieved a porous film which satisfies all the properties of a film such as the strength, permeability, flexibility, appearance, feeling, printing properties, etc.
Besides, a disposal diaper and an absorbent (hereinafter referred to as "diaper and the like") comprise a liquid permeable surface sheet such as a gauze or a non-woven fabric which directly contacts with a skin and a liquid absorbent such as a pulp paper or an absorbent polymer sheet, and a liquid impermeable back sheet used for preventing any leakage of a liquid. Recently, such diaper and the like have become remarkably popular owing to their convenience.
Although a polyethylene film is generally used as the liquid impermeable back sheet, the polyethylene film has no permeability to air and water vapor, and easily becomes stuffy. In order to solve this problem, it has been proposed to use a porous film permeable to water vapor and impermeable to a liquid (Japanese Patent Application Laid-Open (KOKAI) Nos. 58-149303(1983), 60-185803(1985) and 59-69906(1984)). In each of these Japanese Patent Applications Laid-Open (KOKAI), a porous film obtained by kneading polyolefin resin, an inorganic filler and a plasticizer, forming a film from the obtained mixture and stretching the film formed is used. A diaper and the like using such a porous film have an advantage in that the stuffiness is improved.
However, such a porous film has disadvantages in that, according to the kind of the plasticizer used, sufficiently uniform pores are not obtained and stretching spots are produced because of its poor stretching property, the porous film has poor flexibility, feeling and touch, and lacks in tear strength and tensile strength, and an eruption of the skin can occur due to bleeding of a plasticizer on the film surfaces with the passage of time. Therefore, a diaper and the like which can solve all of various problems with respect to the film strength, permeability, appearance and feeling have not been attained.
Further, the following inventions have been proposed: Japanese Patent Applications Laid-Open (KOKAI) Nos. 57-203520(1982), 58-149925(1983), 59-62117(1984), 59-136334(1984), 59-140235(1984), 60-229731(1985), and 60-230825(1985), and Japanese Patent Application No. 62-85219.
As a result of the inventors' studies with a view to solving the above-described problems, it has been found that a porous film obtained by melt-molding a composition composed of polyolefin resin, a filler and an ester of dipentaerythritol having 0 to 70% residual OH groups, and stretching the molded product, resulting in a good balance between the mechanical properties such as tear strength and tensile strength, and permeability to water vapor, and good flexibility, and that a diaper using the above-mentioned porous film as a liquid impermeable back sheet exhibits excellent strength, permeability, appearance and feeling. The present invention has been achieved on the basis of these findings.
It is an object of the present invention to provide a porous sheet which exhibits good extruding properties in melt-molding, particularly excellent stability in molding and which produces a uniaxially stretched product having a good balance between mechanical properties such as tear strength and tensile strength, and permeability to water vapor, uniaxially and biaxially stretched products both having high surface strength and good stretching properties, particularly few stretching spots which occur owing to non-uniform stretching during stretching with a low ratio as well as exhibiting good flexibility.
It is another object of the present invention to provide a disposable diaper and the like which can solve various problems with respect to film strength, permeability, appearance and feeling.